Study Of The Influence Law Of Oxygen Containing Functional Groups On The Floatability Of Low Rank Coal

Low rank coals like brown coal,long flame coal,non-caking coal and weak caking coal,account for 80% of the total coal reserves in China and are the most significant composition of the Chinese coal production and supply.Rational development and utilization of low rank coal is of significant importance to the coal based sustainable energy supply and to the sustainable economic development of China.Since low rank coals are commonly of poor floatability,most of the low rank coals are blended into clean coal instead of being washed.It not only led to the decline in clean coal quality but also caused serious waste of resources and environmental pollution problems.The main reason for the poor floatability of low rank coal is the high oxygen content,especially the existence of organic oxygen which is in the form of oxygen containing functional groups.Oxygen containing functional groups in low rank coal can be divided into three categories,which are single bond carbon oxygen functional groups(hydroxyl and ether),carboxyl and carbonyl.Carboxyl groups and phenolic hydroxyl groups are the most important factors of low rank coal floatability.These two groups are of stronger polarity and hydrogen bond can be formed between them and water molecules.In another way,H3O+ or OH-ions are produced by their ionization or hydrolysis in water and absorbed on the coal surface which made the coal surface charged in the pulp with neutral pH value.Remove the oxygen containing functional groups on the coal surface by surface reaction or mask them by the adsorption of nonpolar groups can significantly improve the hydrophobicity and floatability of low rank coals.The effect of oxygen containing functional groups on the floatability of low rank coal were explained in microscopic point of view by contrasting the masking effect of series of surfactants and the floatability of surface modified coals in this paper.The main conclusions of this paper are as follows.(1)In order to complete the ion exchange reaction,enough ion exchange time is needed in ion exchange method.16 h is enough for carboxyl determination in Pingshuo long flame coal and Shengli brown coal and 24 h is enough for total acidic groups in these two coals.(2)Some organic material in coal could be dissolved by methanol during the wetting process and reduce the oxygen containing functional group content detected by ion exchange method.(3)The solution used for ion exchange method cannot permeate into the low rank coal.The oxygen containing functional groups determined by ion exchange method are oxygen containing functional groups that exists on the coal surface or some groups in the open pores of the coal.Therefore,coal sample for ion exchange method should be limited in a narrow grain size range to make sure that the determined result be representative.(4)Some inorganic impurities in coal can take part in the ion exchange reaction and make the determined result error.The inorganic impurities can be partly removed through the float and sink method and the organic materials in coal would not be changed during the float and sink process.Acid leaching before the ion exchange reaction can also reduce the impurities which make the result of ion exchange method wrong.However,some small molecular organic compounds would be dissolved during the acid leaching process and reduce the oxygen containing functional group content.As a result,acid leaching before ion exchange is not a suitable method for coal purification in ion exchange method.(5)As surfactant being absorbed on low rank coal surface,some oxygen containing functional groups on the coal surface would be masked and the oxygen containing functional group content determined by ion exchange method would be reduced.(6)The masking effect of surfactant on each kind of oxygen containing functional group is different.CTAB,which is a kind of cationic surfactant,is more effective on masking carboxyl than on masking phenolic hydroxyl.The anionic surfactant SDS and nonionic surfactant Span-80 are more effective on masking phenolic hydroxyl than on masking carboxyl.(7)Increasement of the stirring speed and stirring time promote the disperse of coal sample and surfactant in pulp,as a result,more surfactant would be adsorbed on the coal surface and the oxygen containing functional groups on the coal surface were masked more effectively.Compare with the soluble CTAB and SDS,the promoting effect of stirring speed and stirring time is more significant for the modifying process of Span-80.(8)The floatability of Pingshuo long flame coal is determined by its phenolic hydroxyl group content,while the floatability of Shengli brown coal is determined by its total acid group content,and can only be improved when the carboxyl group and phenolic hydroxyl group are both masked.(9)The masking effect of surfactant on oxygen containing functional groups are determined by the strength of the hydrogen bonds between the oxygen containing functional groups and the polar end of the surfactant.There can be more hydrogen bonds between nonionic surfactant Span-80 and the low rank coal surface and the hydrogen bonds between them are stronger than that between low rank coal and CTAB or SDS,so the masking effect of Span-80 on oxygen containing functional groups are more significant.